PROJECT SUMMARY Asymmetric cell division creates daughter cells of different fates, and this is critical for the generation of cellular diversity. Programmed cell death or apoptosis eliminates superfluous cells, and this is critical for cellular homeostasis. Asymmetric cell division and apoptosis are often functionally linked. For example, it is well known that asymmetrically dividing stem cells can control the apoptotic fate of their daughters. However, the molecules and mechanisms involved in coupling these two fundamental processes have so far been elusive. Our long term goal is to understand fully how asymmetric cell division can control apoptosis. To that end we are using the nematode Caenorhabditis elegans as a model system. The NSM neuroblast of C. elegans divides asymmetrically to give rise to a larger cell, the NSM, which survives, and a smaller cell, the NSM sister cell, which undergoes apoptosis. Shortly after cell division, the gene egl-1, which encodes a pro-apoptotic BH3-only protein, is transcriptionally activated in the smaller NSM sister cell, but not the larger NSM, thereby causing apoptosis induction specifically in the NSM sister cell. In the previous funding period, we identified the molecular mechanism that leads to asymmetric egl-1 transcription. We found that it is the absence of the Snail-related transcriptional repressor CES-1 from the NSM sister cell and its presence in the NSM immediately after cell division that causes egl-1 transcription specifically in the NSM sister cell. We have also identified a three component pathway, the dnj- 11/MIDA1, ces-2/HLF, ces-1/Snail pathway, which is required for the asymmetric division of the NSM neuroblast and the exclusion of the CES-1 protein from the NSM sister cell. The overall objective of the current application is to delineate the molecular mechanisms by which the dnj- 11/MIDA1, ces-2/HLF, ces-1/Snail pathway coordinately regulates NSM neuroblast polarity, asymmetric NSM neuroblast division, and the restriction of CES-1 protein to the NSM. Apoptosis is an evolutionary conserved process that plays important roles during embryonic development and in the postembryonic maintenance of cellular homeostasis. Its deregulation has been implicated in a variety of diseases in humans most notably various types of cancers, autoimmune disorders and neurodegenerative diseases;however very little is known thus far about how apoptosis is normally controlled. We discovered that apoptosis can directly be controlled by asymmetric cell division, which represents a new concept in apoptosis regulation. Therefore, the proposed studies will contribute to our basic understanding of developmental health disorders and are specifically relevant to understanding the behavior of cancer stem cells and, hence, tumorigenesis.